Back light and liquid crystal display

BACKGROUND OF THE INVENTION

00011. Field of the Invention

0002The present invention relates to a liquid crystal display and a back light to be arranged behind a liquid crystal panel in the liquid crystal display.

00032. Description of the Related Art

0004Generally, liquid crystal displays have a liquid crystal panel and a back light for irradiating the backside of the liquid crystal panel with light. In recent years, liquid crystal displays have improved and approached CRTs (Cathode Ray Tubes) in terms of performance. Liquid crystal displays thus have found an increasing range of applications, including navigation systems to be mounted on motor vehicles.

0005In motor vehicles, the interior brightness varies greatly between daytime and nighttime hours. Cars are dark inside in the nighttime, so that cars' navigation systems must be lowered sufficiently in screen brightness in the nighttime. That is, liquid crystal displays for use in the navigation systems need to have a smaller minimum luminance for the sake of nighttime use.

0006<figref idref="DRAWINGS">FIG. 1</figref> shows a block diagram of a control circuit in a back light <b>10</b> to be used for this type of liquid crystal display. In the diagram, the back light <b>10</b> includes an oscillating circuit <b>12</b>, lighting circuits <b>14</b><i>a, </i><b>14</b><i>b, </i>and <b>14</b><i>c, </i>a light volume adjusting circuit <b>16</b>, and fluorescent tubes <b>18</b><i>a, </i><b>18</b><i>b, </i>and <b>18</b><i>c. </i>

0007The light volume adjusting circuit <b>16</b> receives brightness adjusting input which is generated in accordance with a luminance adjusting signal from exterior, and outputs, to the oscillating circuit <b>12</b>, a light volume adjusting signal for adjusting the fluorescent tubes <b>18</b><i>a, </i><b>18</b><i>b, </i>and <b>18</b><i>c </i>in brightness. The oscillating circuit <b>12</b> generates an alternating voltage corresponding to the light volume adjusting signal out of the power supplied from a power source, and outputs the generated alternating voltage to the lighting circuits <b>14</b><i>a, </i><b>14</b><i>b, </i>and <b>14</b><i>c. </i>The lighting circuits <b>14</b><i>a, </i><b>14</b><i>b, </i>and <b>14</b><i>c </i>boost the alternating voltage output from the oscillating circuit <b>12</b>, and supply the boosted voltages to the fluorescent tubes <b>18</b><i>a, </i><b>18</b><i>b, </i>and <b>18</b><i>c. </i>The fluorescent tubes <b>18</b><i>a, </i><b>18</b><i>b, </i>and <b>18</b><i>c </i>light up at luminances corresponding to the voltage waveforms supplied.

0008<figref idref="DRAWINGS">FIG. 2</figref> shows an overview of structure of a light emitting part <b>10</b><i>a </i>in the back light <b>10</b>. The light emitting part <b>10</b><i>a </i>has a reflector <b>20</b> for accommodating the fluorescent tubes <b>18</b><i>a, </i><b>18</b><i>b, </i>and <b>18</b><i>c. </i>

0009The inner surface of the reflector <b>20</b> is given a reflecting coat of metal. The fluorescent tubes <b>18</b><i>a, </i><b>18</b><i>b, </i>and <b>18</b><i>c </i>are arranged in parallel inside the reflector <b>20</b>. The light emitted from the fluorescent tubes <b>18</b><i>a, </i><b>18</b><i>b, </i>and <b>18</b><i>c </i>radiates out directly or after reflected from the inner surface of the reflector <b>20</b>.

0010In the back light <b>10</b> shown in <figref idref="DRAWINGS">FIG. 1</figref>, the single light volume adjusting circuit <b>16</b> adjusts the plurality of fluorescent tubes <b>18</b><i>a, </i><b>18</b><i>b, </i>and <b>18</b><i>c </i>in luminance. On this account, when the back light <b>10</b> produces an output of the minimum luminance, the fluorescent tubes <b>18</b><i>a, </i><b>18</b><i>b, </i>and <b>18</b><i>c </i>are lit at their respective minimum luminances. Consequently, the minimum luminance possible for the back light <b>10</b> to output is the sum of the minimum luminances of the individual fluorescent tubes <b>18</b><i>a, </i><b>18</b><i>b, </i>and <b>18</b><i>c. </i>When liquid crystal displays having such a back light are applied to the navigation systems, the screen brightness cannot be lowered to an appropriate brightness in nighttime use.

0011According to the structure of the light emitting part <b>10</b><i>a </i>of the back light <b>10</b> shown in <figref idref="DRAWINGS">FIG. 2</figref>, the central fluorescent tube <b>18</b><i>b </i>faces a smaller area of the reflector <b>20</b> and the outer fluorescent tubes <b>18</b><i>a </i>and <b>18</b><i>b </i>face greater areas of the reflector <b>20</b>. The parasitic capacitance occurring between the fluorescent tube <b>18</b><i>b </i>and the reflector <b>20</b> is therefore smaller than the parasitic capacitance occurring between the fluorescent tube <b>18</b><i>a </i>and the reflector <b>20</b>, and the parasitic capacitance occurring between the fluorescent tube <b>18</b><i>c </i>and the reflector <b>20</b>.

0012Therefore, the current to flow through the fluorescent tube <b>18</b><i>b </i>is greater than the currents to flow the fluorescent tubes <b>18</b><i>a </i>and <b>18</b><i>c. </i>This shortens the life of the fluorescent tube <b>18</b><i>b </i>more than the lives of the fluorescent tubes <b>18</b><i>a </i>and <b>18</b><i>c. </i>In general, fluorescent tubes of a back light cannot be replaced separately. Thus, the entire back light must be replaced when any one of the fluorescent tubes no longer works. That is, the life of a back light becomes shorter depending on the fluorescent tube of the shortest life.

0013Recently, parts of the liquid crystal displays tend to get smaller in size due to a growing demand for liquid crystal panels of larger size. The reflectors accommodating the fluorescent tubes of the back lights also have the inclination to shrink in size. This results in reducing interior spaces of the reflectors and easy trapping of heat within the reflectors. Consequently, if a plurality of fluorescent tubes is used, there is a possibility that concentrate generated heat therein may hamper sufficient heat dissipation.

SUMMARY OF THE INVENTION

0014It is an object of the present invention to provide a back light capable of further reducing the minimum value of luminance and a liquid crystal display equipped with this back light.

0015Another object of the present invention is to prolong the life of the fluorescent tubes constituting the back light.

0016Another object of the present invention is to dissipate heat produced by the light sources constituting the back light with higher efficiency.

0017According to one of the aspects of the back light and the liquid crystal display of the present invention, a brightness control circuit selects at least one of light volume adjusting signals and fixes output of the light volume adjusting signals other than the selected light volume adjusting signal. On this account, the light sources other than the light source corresponding to the selected light volume adjusting signal will not change in luminance. The light sources are arranged, for example, behind a liquid crystal panel.

0018When a luminance adjusting signal changes, the brightness control circuit adjusts the selected light volume adjusting signal and outputs the same to a lighting control circuit so that the luminance of the back light (the total luminance of the light sources) approaches a luminance indicated by the luminance adjusting signal. The lighting control circuit receiving the adjusted light volume adjusting signal generates a voltage having a waveform corresponding to the received light volume adjusting signal and applies the same to the light source. The light source is on at a luminance corresponding to the waveform of the voltage applied from the lighting control circuit. Then, the light source corresponding to the selected light volume adjusting signal is adjusted in luminance, so that the back light changes in luminance. Subsequently, the brightness control circuit fixes the output of the selected light volume adjusting signal and selects one or more of the light volume adjusting signals according to need. Then, the same luminance adjustment as described above is performed.

0019The back light of the present invention can adjust the light volume adjusting signals one by one to change the luminances of the light sources, whereby the luminance adjustment control of the back light is facilitated. Since the plurality of light sources can be successively adjusted in luminance, minimum light sources alone can be turned on at their respective minimum luminances with the rest of the light sources turned off. For example, turning only a single light source on at its minimum luminance enables the minimum luminance of a single light source to be the minimum luminance of the back light. Consequently, when the back light and the liquid crystal display of the present invention are used in dark places, the screen brightness can be lowered to an appropriate brightness.

0020According to another aspect of the back light of the present invention, an adjusting part adjusts the selected light volume adjusting signal to lower its corresponding light source in luminance (luminance-down operation) when the luminance indicated by the luminance adjusting signal is smaller than the actual total luminance of the light sources. A detecting part outputs a detecting signal to the adjusting part when the luminance of the light source corresponding to the selected light volume adjusting signal is minimum.

0021On receiving the detecting signal, the adjusting part fixes the selected light volume adjusting signal to a turn-off level to turn off the corresponding light source (turn-off operation), and selects anew at least one of the rest of the light volume adjusting signals corresponding to a lit light source (selecting operation). In this state, when the luminance indicated by the luminance adjusting signal is smaller than the actual luminance of the back light, the adjusting part adjusts the newly selected light volume adjusting signal to lower the corresponding light source in luminance.

0022The adjusting part thus performs the luminance-down operation, turn-off operation, and selecting operation in succession to adjust the luminance of the back light. Since the reception of the detecting signal can facilitate detecting that the light source has reached its minimum luminance, the control of lowering the luminance of the back light can be simplified.

0023According to another aspect of the back light of the present invention, the adjusting part temporarily adjusts the light volume adjusting signal corresponding to at least one of the lit light sources in synchronization with the selected light volume adjusting signal's turning to the turn-off level, thereby enhancing the corresponding light source in luminance temporarily. Consequently, the back light can be prevented from dropping in luminance when a light source lit at its minimum luminance is turned off.

0024According to another aspect of the back light of the present invention, the detecting part includes a monitoring circuit for directly detecting currents supplied to the light sources. The detecting part can thus detect the luminances of the light sources with higher precision. That is, the detecting part can detect more precisely that light sources are lit at the minimum luminances. This allows the detecting part to output the detecting signal to the adjusting part with more accurate timing. As a result, the detecting part can perform precise control of lowering the luminance of the back light.

0025According to another aspect of the back light of the present invention, a fluorescent tube causing a smaller parasitic capacitance with the reflector is subjected to an effective voltage relatively lower than that applied to the rest of the fluorescent tubes. Consequently, the fluorescent tube with smaller parasitic capacitance is supplied with a current smaller than the rest of the fluorescent tubes are. The fluorescent tube with smaller parasitic capacitance leaks a current smaller than the rest of the fluorescent tubes do, resulting in equalizing the currents which contribute to light emission of the fluorescent tubes. The lives of fluorescent tubes can thus be made equal to each other. As a result, the life of the back light can be prolonged.

0026According to another aspect of the back light of the present invention, a fluorescent tube causing a smaller parasitic capacitance with the reflector is smaller in diameter than the rest of the fluorescent tubes, and this is the smaller the diameter, the higher the impedance. Consequently, given that the same effective voltage is applied to all the fluorescent tubes, the fluorescent tube with higher impedance is supplied with a current smaller than the rest of the fluorescent tubes are. Here, the fluorescent tube with higher impedance leaks a current smaller than the rest of the fluorescent tubes do, so that the amounts of the currents contributing to light emission of the fluorescent tubes become equal to each other. This enables the fluorescent tubes to be identical in life. The life of the back light can be prolonged accordingly.

0027According to another aspect of the back light of the present invention, the fluorescent tube causing a smaller parasitic capacitance with the reflector is higher in internal gas pressure than the rest of the fluorescent tubes, and this is the smaller the internal gas pressure, the higher the impedance. Consequently, given that the same effective voltage is applied to all the fluorescent tubes, the fluorescent tube with higher impedance is supplied with a current smaller than the rest of the fluorescent tubes are. Here, the fluorescent tube with higher impedance leaks a current smaller than the rest of the fluorescent tubes do so that the amounts of the currents contributing to light emission of the fluorescent tubes become equal to each other. This enables the lives of the fluorescent tubes to be identical. The life of the back light can be prolonged accordingly.

0028According to another aspect of the back light of the present invention, each reflector accommodates a single light source. This allows dissipation of heat produced by the light sources with higher efficiency.

0029In addition, the parasitic capacitances occurring between the light sources and the respective reflectors can be substantially equalized so that the amounts of the current leakage from the light sources to the respective reflectors can be also equalized. Accordingly, the amounts of the currents contributing to light emission of the light sources become equal to each other, whereby the light sources can be made identical in life. As a result, the life of the backlight can be prolonged.

BRIEF DESCRIPTION OF THE DRAWINGS

0030The nature, principle, and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by identical reference numbers, in which:

0031<figref idref="DRAWINGS">FIG. 1</figref> is a block diagram of the control circuit in a conventional back light;

0032<figref idref="DRAWINGS">FIG. 2</figref> is an explanatory diagram showing an overview of the structure of a light emitting part in the conventional back light;

0033<figref idref="DRAWINGS">FIG. 3</figref> is a block diagram showing a structure of the liquid crystal display according to a first embodiment of the present invention;

0034<figref idref="DRAWINGS">FIG. 4</figref> is a block diagram showing the details of control in the back light of <figref idref="DRAWINGS">FIG. 3</figref>;

0035<figref idref="DRAWINGS">FIG. 5</figref> is an explanatory diagram showing luminance variations of the individual fluorescent tubes and the back light of the first embodiment;

0036<figref idref="DRAWINGS">FIG. 6</figref> is a block diagram showing a structure of the liquid crystal display according to a second embodiment of the present invention;

0037<figref idref="DRAWINGS">FIG. 7</figref> is a block diagram showing the details of control in the back light of <figref idref="DRAWINGS">FIG. 6</figref>;

0038<figref idref="DRAWINGS">FIG. 8</figref> is an explanatory diagram showing luminance variations of the individual fluorescent tubes and the back light of the second embodiment;

0039<figref idref="DRAWINGS">FIG. 9</figref> is a block diagram showing a structure of the liquid crystal display according to a third embodiment of the present invention;

0040<figref idref="DRAWINGS">FIG. 10</figref> is a block diagram showing the details of control in the back light of <figref idref="DRAWINGS">FIG. 9</figref>;

0041<figref idref="DRAWINGS">FIG. 11</figref> is an explanatory diagram showing luminance variations of the individual fluorescent tubes and the back light of the third embodiment;

0042<figref idref="DRAWINGS">FIG. 12</figref> is a block diagram showing a fourth embodiment of the liquid crystal display of the present invention; and

0043<figref idref="DRAWINGS">FIG. 13</figref> is a block diagram showing a structural example of the back light in which the fluorescent tubes are disposed on only one side of a light guide plate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

0044Hereinafter, embodiments of the present invention will be described with reference to the drawings.

0045<figref idref="DRAWINGS">FIG. 3</figref> shows a first embodiment of the liquid crystal display of the present invention. In the diagram, a liquid crystal display <b>28</b> includes a liquid crystal panel <b>30</b>, a back light <b>32</b>, and a case <b>34</b> for accommodating the liquid crystal panel <b>30</b> and the back light <b>32</b>.

0046The liquid crystal panel <b>30</b> has a structure that liquid crystal is sealed between two transparent glass plates which are opposed to each other with a predetermined spacing. The liquid crystal panel <b>30</b> is of TN (Twisted Nematic) type, for example. The liquid crystal panel <b>30</b> is not limited to TN type but may be of, e.g., VA (Vertically Aligned) type.

0047The back light <b>32</b> includes a prism sheet <b>36</b>, a diffusion sheet <b>38</b>, a light guide plate <b>40</b> (light pipe), a reflecting sheet <b>42</b>, and light emitting parts <b>44</b>A arranged on both sides of the light guide plate <b>40</b>, respectively, as well as a brightness control circuit <b>48</b> and lighting control circuits <b>50</b><i>a, </i><b>50</b><i>b, </i>and <b>50</b><i>c </i>which are shown in <figref idref="DRAWINGS">FIG. 4</figref> to be seen later. The light emitting parts <b>44</b>A contain fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c </i>each.

0048The prism sheet <b>36</b> is arranged behind the liquid crystal panel <b>30</b>. The diffusion sheet <b>38</b> is arranged behind the prism sheet <b>36</b>. The light guide plate <b>40</b> is arranged behind the diffusion sheet <b>38</b> with its light emitting side toward the diffusion sheet <b>38</b>. The light guide plate <b>40</b> is made of transparent synthetic resin such as acrylic, and has a generally flat shape. The reflecting sheet <b>42</b> is arranged on the back of the light guide plate <b>40</b>.

0049The light emitting parts <b>44</b>A have a reflector <b>45</b> of U-shaped section. The light emitting parts <b>44</b>A are arranged with the apertures of the respective reflectors <b>45</b> toward sides of the light guide plate <b>40</b>. The inner surfaces of the reflectors <b>45</b> are provided with a reflecting part <b>52</b> (reflecting coat) for reflecting light emitted from the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c. </i>Incidentally, the reflectors <b>45</b> are not limited to the foregoing configuration. For example, reflectors of semicircular section may be used.

0050The fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c </i>are identical to one another. The fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c </i>are arranged at regular intervals in the respective reflectors <b>45</b> so as to face the sides of the light guide plate <b>40</b>. The fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c </i>are subjected to voltages from the lighting control circuits <b>50</b><i>a, </i><b>50</b><i>b, </i>and <b>50</b><i>c </i>to be described later, and light up at luminances corresponding to the waveforms of these voltages. The minimum luminances possible for the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c </i>to light at with stability are approximately 20% the maximum luminances.

0051In the present embodiment, luminance adjustment control of the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c </i>accommodated in one of the light emitting parts <b>44</b>A and of the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c </i>accommodated in the other light emitting part <b>44</b>A are performed in synchronization with each other. For this reason, the following description will exclusively deal with the luminance adjustment control of the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c </i>accommodated in one of the light emitting parts <b>44</b>A.

0052<figref idref="DRAWINGS">FIG. 4</figref> shows the details of control in the back light <b>32</b>. The back light <b>32</b> has the brightness control circuit <b>48</b>, the lighting control circuits <b>50</b><i>a, </i><b>50</b><i>b, </i>and <b>50</b><i>c, </i>an oscillating circuit <b>54</b>, and the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c. </i>The brightness control circuit <b>48</b> is composed of a detecting part <b>56</b> and an adjusting part <b>58</b>.

0053The detecting part <b>56</b> includes a monitoring circuit <b>60</b>. The monitoring circuit <b>60</b> detects the currents supplied to the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c, </i>and outputs the luminance of the back light <b>32</b> (the total luminance of the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c</i>) determined from the detected currents to the adjusting part <b>58</b> as a luminance detecting signal VD. The monitoring circuit <b>60</b> outputs a detecting signal DET<b>1</b> to the adjusting part <b>58</b> when it detects that a fluorescent tube (any of <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c</i>) reaches a minimum luminance.

0054The adjusting part <b>58</b> has a comparing unit <b>62</b>, a selecting unit <b>64</b>, and signal generating units <b>66</b><i>a, </i><b>66</b><i>b, </i>and <b>66</b><i>c. </i>The comparing unit <b>62</b> receives a luminance adjusting signal LUM for adjusting the luminance of the back light <b>32</b> from exterior. The comparing unit <b>62</b> also receives the luminance detecting signal VD which indicates the luminance of the back light <b>32</b>. The comparing unit <b>62</b> outputs a gap between the luminance indicated by the luminance adjusting signal LUM and the luminance of the back light <b>32</b> indicated by the luminance gap signal GAP. The comparing unit <b>62</b> also outputs a detecting signal DET<b>2</b> to the selecting unit <b>64</b> in synchronization with the detecting signal DET<b>1</b>.

0055The selecting unit <b>64</b> outputs any one of selecting signals OPTa, OPTb, and OPTc all the time (always keeps any one of them at high level). The selecting signal OPTa is output to the signal generating unit <b>66</b><i>a. </i>The selecting signal OPTb is output to the signal generating unit <b>66</b><i>b. </i>The selecting signal OPTc is output to the signal generating unit <b>66</b><i>c. </i>

0056The selecting unit <b>64</b> outputs the selecting signal OPTa when the back light <b>32</b> is at the maximum in luminance. Upon receiving the detecting signal DET<b>2</b> while outputting the selecting signal OPTa, the selecting unit <b>64</b> stops outputting the selecting signal OPTa (turns the selecting signal OPTa to low level) in synchronization with the reception and outputs the selecting signal OPTb (turns the selecting signal OPTb to high level). Upon receiving the detecting signal DET<b>2</b> while outputting the selecting signal OPTb, the selecting unit <b>64</b> stops outputting the selecting signal OPTb (turns the selecting signal OPTb to low level) in synchronization with the reception and outputs the selecting signal OPTc (turns the selecting signal OPTc to high level). Upon receiving the detecting signal DET<b>2</b> while outputting the selecting signal OPTc, the selecting unit <b>64</b> keeps outputting the selecting signal OPTc without any change in output.

0057The signal generating unit <b>66</b><i>a, </i>while receiving the selecting signal OPTa of high level, operates to adjust the fluorescent tube <b>46</b><i>a </i>in luminance. More specifically, the signal generating unit <b>66</b><i>a </i>outputs, to the lighting control circuit <b>50</b><i>a, </i>a light volume adjusting signal ADJa for adjusting the luminance of the fluorescent tube <b>46</b><i>a </i>so that the luminance gap signal GAP decreases. Moreover, in synchronization with the falling edge of the selecting signal OPTa, the signal generating unit <b>66</b><i>a </i>fixes the light volume adjusting signal ADJa to a turn-off level for turning off the fluorescent tube <b>46</b><i>a. </i>

0058The signal generating unit <b>66</b><i>b, </i>while receiving the selecting signal OPTb of high level, operates to adjust the fluorescent tube <b>46</b><i>b </i>in luminance. More specifically, the signal generating unit <b>66</b><i>b </i>outputs, to the lighting control circuit <b>50</b><i>b, </i>a light volume adjusting signal ADJb for adjusting the luminance of the fluorescent tube <b>46</b><i>b </i>so that the luminance gap signal GAP decreases. Moreover, in synchronization with the falling edge of the selecting signal OPTb, the signal generating unit <b>66</b><i>b </i>fixes the light volume adjusting signal ADJb to a turn-off level for turning off the fluorescent tube <b>46</b><i>b. </i>

0059The signal generating unit <b>66</b><i>c, </i>while receiving the selecting signal OPTc of high level, operates to adjust the fluorescent tube <b>46</b><i>c </i>in luminance. More specifically, the signal generating unit <b>66</b><i>c </i>outputs, to the lighting control circuit <b>50</b><i>c, </i>a light volume adjusting signal ADJc for adjusting the luminance of the fluorescent tube <b>46</b><i>c </i>so that the luminance gap signal GAP decreases.

0060The signal generating units <b>66</b><i>a, </i><b>66</b><i>b, </i>and <b>66</b><i>c </i>fix the output of the respective light volume adjusting signals ADJa, ADJb, and ADJc unless receiving the selecting signals OPTa, OPTb, and OPTc (while the selecting signals OPTa, OPTb, and OPTc are at low level).

0061Now, the light volume adjusting signal ADJb to be output from the signal generating unit <b>66</b><i>b </i>is set to be relatively smaller than the light volume adjusting signal ADJa and the light volume adjusting signal ADJc. On this account, the effective voltage that the lighting control circuit <b>50</b><i>b </i>applies to the fluorescent tube <b>46</b><i>b </i>is relatively lower than the effective voltage that the lighting control circuit <b>50</b><i>a </i>applies to the fluorescent tube <b>46</b><i>a </i>and the effective voltage that the lighting control circuit <b>50</b> applies to the fluorescent tube <b>46</b><i>c. </i>

0062To put it in details, the fluorescent tube <b>46</b><i>b </i>arranged in the center of the reflector <b>45</b> shown in <figref idref="DRAWINGS">FIG. 3</figref> faces a smaller area of the reflecting part <b>52</b> than the outer fluorescent tubes <b>46</b><i>a </i>and <b>46</b><i>c </i>do. That is, the parasitic capacitance occurring between the fluorescent tube <b>46</b><i>b </i>and the reflector <b>45</b> is smaller than the parasitic capacitance occurring between the fluorescent tube <b>46</b><i>a </i>and the reflector <b>45</b> and the parasitic capacitance occurring between the fluorescent tube <b>46</b><i>c </i>and the reflector <b>45</b>. This makes the leakage current from the fluorescent tube <b>46</b><i>b </i>to the reflector <b>45</b> smaller than the leakage currents from the fluorescent tubes <b>46</b><i>a </i>and <b>46</b><i>c </i>to the reflector <b>45</b>. When the effective voltage to be applied to the fluorescent tube <b>46</b><i>b </i>is made relatively lower than the effective voltages to be applied to the fluorescent tubes <b>46</b><i>a </i>and <b>46</b><i>c </i>as mentioned above, the current supplied to the fluorescent tube <b>46</b><i>b </i>becomes smaller than the currents supplied to the fluorescent tubes <b>46</b><i>a </i>and <b>46</b><i>c. </i>Here, the current leaking from the fluorescent tube <b>46</b><i>b </i>of smaller parasitic capacitance to the reflector <b>45</b> is smaller than the currents leaking from the fluorescent tubes <b>46</b><i>a </i>and <b>46</b><i>c. </i>Then, the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c </i>turn out to be identical in the current that contributes to light emission.

0063The lighting control circuit <b>50</b><i>a </i>is formed for the fluorescent tube <b>46</b><i>a, </i>and has a light volume adjusting circuit <b>70</b><i>a </i>and a lighting circuit <b>72</b><i>a. </i>The lighting control circuit <b>50</b><i>b </i>is formed for the fluorescent tube <b>46</b><i>b, </i>and has a light volume adjusting circuit <b>70</b><i>b </i>and a lighting circuit <b>72</b><i>b. </i>The lighting control circuit <b>50</b><i>c </i>is formed for the fluorescent tube <b>46</b><i>c, </i>and has a light volume adjusting circuit <b>70</b><i>c </i>and a lighting circuit <b>72</b><i>c. </i>

0064The light volume adjusting circuits <b>70</b><i>a, </i><b>70</b><i>b, </i>and <b>70</b><i>c </i>are supplied with an alternating voltage from the oscillating circuit <b>54</b>. The light volume adjusting circuit <b>70</b><i>a </i>receives the light volume adjusting signal ADJa, adjusts the alternating voltage in amplitude according to the luminance indicated by the light volume adjusting signal ADJa, and outputs the adjusted alternating voltage to the lighting circuit <b>72</b><i>a. </i>The light volume adjusting circuit <b>70</b><i>b </i>receives the light volume adjusting signal ADJb, adjusts the alternating voltage in amplitude according to the luminance indicated by the light volume adjusting signal ADJb, and outputs the adjusted alternating voltage to the lighting circuit <b>72</b><i>b. </i>The light volume adjusting circuit <b>70</b><i>c </i>receives the light volume adjusting signal ADJc, adjusts the alternating voltage in amplitude according to the luminance indicated by the light volume adjusting signal ADJc, and outputs the adjusted alternating voltage to the lighting circuit <b>72</b><i>c. </i>

0065Each of the lighting circuits <b>72</b><i>a, </i><b>72</b><i>b, </i>and <b>72</b><i>c </i>boosts the alternating voltage supplied from the light volume adjusting circuit and applies the boosted voltage to the corresponding fluorescent tube (any of <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c</i>).

0066Incidentally, in the present embodiment, the alternative voltages are adjusted in amplitude (effective value) to adjust the luminances of the respective fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c. </i>Otherwise, the alternative voltages to be applied to the fluorescent tubes may be adjusted in period (duty ratio control) to adjust the luminances of the respective fluorescent tubes.

0067Now, description will be given of the luminance adjustment of the back light <b>32</b> described above.

0068<figref idref="DRAWINGS">FIG. 5</figref> shows luminance variations of the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c, </i>and the back light <b>32</b>. In this example, in an initial state (at time t<b>0</b>), the signal generating units <b>66</b><i>a, </i><b>66</b><i>b, </i>and <b>66</b><i>c </i>shown in <figref idref="DRAWINGS">FIG. 4</figref> output light volume adjusting signals ADJa, ADJb, and ADJc for lighting the corresponding fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c </i>at the maximum luminances, respectively. The selecting unit <b>64</b> outputs the selecting signal OPTa to select the signal generating unit <b>66</b><i>a. </i>The fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c </i>are lit at their respective maximum luminances. That is, at time t<b>0</b>, the back light <b>32</b> is at the maximum in luminance.

0069In this example, the comparing unit <b>62</b> receives, at time t<b>1</b>, a luminance adjusting signal LUM indicating that the back light <b>32</b> is to be minimized in luminance. Here, the luminance indicated by the luminance adjusting signal LUM is smaller than the luminance indicated by the luminance detecting signal VD. The comparing unit <b>62</b> thus outputs a luminance gap signal GAP for lowering the luminance of the back light. The signal generating unit <b>66</b><i>a </i>is in a luminance adjustable state, receiving the selecting signal OPTa of high level. The signal generating unit <b>66</b><i>a </i>outputs to the light volume adjusting circuit <b>70</b><i>a </i>a light volume adjusting signal ADJa for lowering the luminance of the fluorescent tube <b>46</b><i>a </i>(luminance-down operation).

0070Consequently, the fluorescent tube <b>46</b><i>a </i>gradually decreases in luminance (<figref idref="DRAWINGS">FIG. 5(</figref><i>a</i>)). With the decreasing luminance of the fluorescent tube <b>46</b><i>a, </i>the back light <b>32</b> also decreases in luminance (<figref idref="DRAWINGS">FIG. 5(</figref><i>b</i>)). Since the output of the signal generating units <b>66</b><i>b </i>and <b>66</b><i>c </i>is fixed, the fluorescent tubes <b>46</b><i>b </i>and <b>46</b><i>c </i>have no change in luminance (<figref idref="DRAWINGS">FIG. 5(</figref><i>c</i>)).

0071At time t<b>2</b>, the luminance of the fluorescent tube <b>46</b><i>a </i>reaches the minimum luminance, and the detecting part <b>56</b> outputs the detecting signal DET<b>1</b> (<figref idref="DRAWINGS">FIG. 5(</figref><i>d</i>)). In synchronization with the reception of the detecting signal DET<b>1</b>, the comparing unit <b>62</b> outputs the detecting signal DET<b>2</b> (<figref idref="DRAWINGS">FIG. 5(</figref><i>d</i>)). In response to the detecting signal DET<b>2</b>, the selecting unit <b>64</b> turns the selecting signal OPTa to low level and turns the selecting signal OPTb to high level. In synchronization with the falling edge of the selecting signal OPTa, the signal generating unit <b>66</b><i>a </i>fixes the light volume adjusting signal ADJa to the turn-off level for turning off the fluorescent tube <b>46</b><i>a </i>(turn-off operation). Then, the fluorescent tube <b>46</b><i>a </i>goes out (<figref idref="DRAWINGS">FIG. 5(</figref><i>e</i>)). The signal generating unit <b>66</b><i>b </i>receives the selecting signal OPTb of high level and enters a luminance adjustable state. The signal generating unit <b>66</b><i>b </i>outputs to the light volume adjusting circuit <b>70</b><i>b </i>a light volume adjusting signal ADJb for lowering the luminance of the fluorescent tube <b>46</b><i>b </i>(luminance-down operation). Consequently, the fluorescent tube <b>46</b><i>b </i>decreases in luminance (<figref idref="DRAWINGS">FIG. 5(</figref><i>f</i>)). With the decreasing luminance of the fluorescent tube <b>46</b><i>b, </i>the back light <b>32</b> also decreases in luminance (<figref idref="DRAWINGS">FIG. 5(</figref><i>g</i>)).

0072At time t<b>3</b>, the luminance of the fluorescent tube <b>46</b><i>b </i>reaches the minimum luminance, and the detecting part <b>56</b> outputs the detecting signal DET<b>1</b> (<figref idref="DRAWINGS">FIG. 5(</figref><i>h</i>)). In synchronization with the detecting signal DET<b>1</b>, the comparing unit <b>62</b> outputs the detecting signal DET<b>2</b> (<figref idref="DRAWINGS">FIG. 5(</figref><i>h</i>)). In response to the detecting signal DET<b>2</b>, the selecting unit <b>64</b> turns the selecting signal OPTb to low level and turns the selecting signal OPTc to high level. In synchronization with the falling edge of the selecting signal OPTb, the signal generating unit <b>66</b><i>b </i>fixes the light volume adjusting signal ADJb to the turn-off level for turning off the fluorescent tube <b>46</b><i>b </i>(turn-off operation). Then, the fluorescent tube <b>46</b><i>b </i>goes out (<figref idref="DRAWINGS">FIG. 5(</figref><i>i</i>)). The signal generating unit <b>66</b><i>c </i>receives the selecting signal OPTc of high level and enters a luminance adjustable state. The signal generating unit <b>66</b><i>c </i>outputs to the light volume adjusting circuit <b>70</b><i>c </i>a light volume adjusting signal ADJc for lowering the luminance of the fluorescent tube <b>46</b><i>c </i>(luminance-down operation). Consequently, the fluorescent tube <b>46</b><i>c </i>decreases in luminance (<figref idref="DRAWINGS">FIG. 5(</figref><i>j</i>)). With the decreasing luminance of the fluorescent tube <b>46</b><i>c, </i>the back light <b>32</b> also decreases in luminance (<figref idref="DRAWINGS">FIG. 5(</figref><i>k</i>)).

0073At time t<b>4</b>, the luminance of the fluorescent tube <b>46</b><i>c </i>reaches the minimum luminance, and the detecting part <b>56</b> outputs the detecting signal DET<b>1</b> (<figref idref="DRAWINGS">FIG. 5(</figref><i>l</i>)). In synchronization with the detecting signal DET<b>1</b>, the comparing unit <b>62</b> outputs the detecting signal DET<b>2</b> (<figref idref="DRAWINGS">FIG. 5(</figref><i>l</i>)). Fluorescent tubes lit here are the fluorescent tube <b>46</b><i>c </i>alone. That is, at time t<b>4</b>, the back light <b>32</b> becomes minimum in luminance. Here, the luminance indicated by the luminance detecting signal VD becomes equal to the luminance indicated by the luminance adjusting signal LUM, and the comparing unit <b>62</b> outputs a luminance gap signal GAP of 0 in magnitude. When the fluorescent tube <b>46</b><i>c </i>lit to the end becomes minimum in luminance, the brightness control circuit <b>48</b> makes no turn-off operation and keeps the fluorescent tube <b>46</b><i>c </i>lit at the minimum luminance.

0074In a case where the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c </i>are lit at their maximum luminances, the luminance of the back light <b>32</b> is maximum. Also, when only a single fluorescent tube (<b>46</b><i>c, </i>in this example) in each light emitting part <b>44</b>A is lit at its minimum luminance, the luminance of the back light <b>32</b> is minimum. Hence, the minimum luminance of the back light <b>32</b> is one-third that of conventional back lights in which all the fluorescent tubes are lit together at their minimum luminances.

0075As has been described, according to the present embodiment, the adjusting part <b>58</b> has only to receive the detecting signal DET<b>1</b> to easily detect that any one of the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c </i>reaches its minimum luminance. Thus, the adjusting part <b>58</b> can easily switch the fluorescent tube to adjust (any one of <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c</i>). This can facilitate the control in lowering the luminance of the back light <b>32</b>.

0076Since the detecting part <b>56</b> directly detects the currents supplied to the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c </i>by using the monitoring circuit <b>60</b>, it can easily detect that any one of the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c </i>reaches the minimum luminance.

0077The fluorescent tube <b>46</b><i>b, </i>which causes a smaller parasitic capacitance with the reflector <b>45</b>, is subjected to an effective voltage relatively lower than the effective voltages applied to the other fluorescent tubes <b>46</b><i>a </i>and <b>46</b><i>c. </i>This can make the florescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c </i>identical in the current that contributes to light emission. The fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c </i>can thus be made identical in life. The result is that the fluorescent tube <b>46</b><i>b, </i>causing a smaller parasitic capacitance with the reflector <b>45</b>, is prolonged in life, allowing extended life of the back light <b>32</b>.

0078<figref idref="DRAWINGS">FIG. 6</figref> shows a second embodiment of the liquid crystal display of the present invention. The same elements as those of the first embodiment will be designated by identical reference numbers or symbols. Detailed description will be omitted of these elements. A liquid crystal display <b>28</b><i>b </i>includes a liquid crystal panel <b>30</b>, a back light <b>32</b><i>b, </i>and a case <b>34</b> for accommodating the liquid crystal panel <b>30</b> and the back light <b>32</b><i>b. </i>

0079The back light <b>32</b><i>b </i>includes a prism sheet <b>36</b>, a diffusion sheet <b>38</b>, a light guide plate <b>40</b>, a reflecting sheet <b>42</b>, light emitting parts <b>44</b>B arranged on both sides of the light guide plate <b>40</b>, a brightness control circuit <b>48</b><i>b, </i>and lighting control circuits <b>50</b><i>a, </i><b>50</b><i>b, </i>and <b>50</b><i>c </i>which are shown in <figref idref="DRAWINGS">FIG. 5</figref> and to be described later.

0080The back light <b>32</b><i>b </i>has the same configuration as that of the first embodiment except that the diameters of fluorescent tubes <b>46</b><i>d </i>arranged in the centers of reflectors <b>45</b> of the light emitting parts <b>44</b>B are smaller than those of fluorescent tubes <b>46</b><i>a </i>and <b>46</b><i>c </i>arranged on both sides of the respective fluorescent tubes <b>46</b><i>d, </i>and that signal generating units <b>66</b><i>d </i>and <b>66</b><i>e </i>(shown in <figref idref="DRAWINGS">FIG. 7</figref> to be seen later) are formed instead of the signal generating units <b>66</b><i>b </i>and <b>66</b><i>c </i>of the first embodiment.

0081The fluorescent tubes <b>46</b><i>d </i>are smaller in diameter than the fluorescent tubes <b>46</b><i>a </i>and <b>46</b><i>c, </i>and thus are higher in impedance than the fluorescent tubes <b>46</b><i>a </i>and <b>46</b><i>c. </i>Accordingly, given that the same effective voltage is applied to all the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>d, </i>and <b>46</b><i>c, </i>the currents supplied to the fluorescent tubes <b>46</b><i>d </i>of higher impedance become smaller than the currents supplied to the other fluorescent tubes <b>46</b><i>a </i>and <b>46</b><i>c. </i>Moreover, as in the first embodiment, the fluorescent tubes <b>46</b><i>d </i>arranged in the centers of the reflectors <b>45</b> are less prone to leak currents to the reflectors <b>45</b> than the fluorescent tubes <b>46</b><i>a </i>and <b>46</b><i>c </i>do. Consequently, the currents leaking from the fluorescent tubes <b>46</b><i>d </i>of higher impedance to the reflector <b>45</b> become smaller than the currents leaking from the fluorescent tubes <b>46</b><i>a </i>and <b>46</b><i>c. </i>Then, the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>d, </i>and <b>46</b><i>c </i>turn out to be identical in the current that contributes to light emission. Thus, as in the first embodiment, the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>d, </i>and <b>46</b><i>c </i>become identical in life.

0082In the present embodiment, the lighting control circuits <b>50</b><i>a, </i><b>50</b><i>b, </i>and <b>50</b><i>c </i>shown in <figref idref="DRAWINGS">FIG. 5</figref> to be seen later apply relatively the same effective voltages to the respective fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>d, </i>and <b>46</b><i>c. </i>

0083Now, in the present embodiment, luminance adjustment control of the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>d, </i>and <b>46</b><i>c </i>accommodated in one of the light emitting parts <b>44</b>B and luminance adjustment control of the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>d, </i>and <b>46</b><i>c </i>accommodated in the other light emitting part <b>44</b>B are performed in synchronization with each other. For this reason, the following description will exclusively deal with the luminance adjustment control of the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>d, </i>and <b>46</b><i>c </i>accommodated in one of the light emitting parts <b>44</b>B.

0084<figref idref="DRAWINGS">FIG. 7</figref> shows the details of control of the back light <b>32</b><i>b. </i>

0085The signal generating unit <b>66</b><i>d </i>receives a luminance gap signal GAP and a selecting signal OPTb, and outputs to the lighting control circuit <b>50</b><i>b </i>a light volume adjusting signal ADJd for adjusting the luminance of the fluorescent tube <b>46</b><i>d. </i>In response to the rising edge of the selecting signal OPTb, the signal generating unit <b>66</b><i>d </i>enhances the light volume adjusting signal ADJd steeply so that the fluorescent tube <b>46</b><i>d </i>increases in luminance as much as the minimum luminance of the fluorescent tube <b>46</b><i>a. </i>Subsequently, while receiving the selecting signal OPTb of high level, the signal generating unit <b>66</b><i>d </i>outputs to the lighting control circuit <b>50</b><i>b </i>a light volume adjusting signal ADJd for adjusting the luminance of the fluorescent tube <b>46</b><i>d </i>so that the luminance gap signal GAP decreases. Besides, in synchronization with the falling edge of the selecting signal OPTb, the signal generating unit <b>66</b><i>d </i>fixes the light volume adjusting signal ADJd to a turn-off level for turning off the fluorescent tube <b>46</b><i>d. </i>

0086The signal generating unit <b>66</b><i>e </i>receives the luminance gap signal GAP and a selecting signal OPTc, and outputs to the lighting control circuit <b>50</b><i>c </i>a light volume adjusting signal ADJe for adjusting the luminance of the fluorescent tube <b>46</b><i>c. </i>In response to the rising edge of the selecting signal OPTc, the signal generating unit <b>66</b><i>e </i>enhances the light volume adjusting signal ADJe steeply so that the fluorescent tube <b>46</b><i>c </i>increases in luminance as much as the minimum luminance of the fluorescent tube <b>46</b><i>d. </i>Subsequently, while receiving the selecting signal OPTc of high level, the signal generating unit <b>66</b><i>e </i>outputs to the lighting control circuit <b>50</b><i>c </i>a light volume adjusting signal ADJe for adjusting the luminance of the fluorescent tube <b>46</b><i>c </i>so that the luminance gap signal GAP decreases.

0087Incidentally, the signal generating units <b>66</b><i>d </i>and <b>66</b><i>e </i>fix the output of the light volume adjusting signals ADJd and ADJe while the selecting signals OPTb and OPTc are at low level.

0088Now, description will be given of the luminance adjustment of the back light <b>32</b><i>b </i>described above.

0089<figref idref="DRAWINGS">FIG. 8</figref> shows luminance variations of the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>d, </i>and <b>46</b><i>c, </i>and the back light <b>32</b><i>b. </i>In this example, as with the first embodiment, the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>d, </i>and <b>46</b><i>c </i>are lit at their respective maximum luminances in an initial state (at time t<b>0</b>). In addition, the selecting unit <b>64</b> outputs the selecting signal OPTa of high level and keeps the selecting signals OPTb and OPTc at low level.

0090At time t<b>1</b>, a luminance adjusting signal LUM indicating that the back light <b>32</b><i>b </i>is to be minimized in luminance is supplied, in which case the fluorescent tube <b>46</b><i>a </i>gradually decreases in luminance (<figref idref="DRAWINGS">FIG. 8(</figref><i>a</i>)). With the decreasing luminance of the fluorescent tube <b>46</b><i>a, </i>the back light <b>32</b><i>b </i>also decreases in luminance (<figref idref="DRAWINGS">FIG. 8(</figref><i>b</i>)). Since the output of the signal generating units <b>66</b><i>d </i>and <b>66</b><i>e </i>is fixed, the fluorescent tubes <b>46</b><i>d </i>and <b>46</b><i>c </i>have no change in luminance (<figref idref="DRAWINGS">FIG. 8(</figref><i>c</i>)).

0091At time t<b>2</b>, the luminance of the fluorescent tube <b>46</b><i>a </i>reaches the minimum luminance, and the detecting part <b>56</b> outputs the detecting signal DET<b>1</b> (<figref idref="DRAWINGS">FIG. 8(</figref><i>d</i>)). In synchronization with the detecting signal DET<b>1</b>, the comparing unit <b>62</b> outputs the detecting signal DET<b>2</b> (<figref idref="DRAWINGS">FIG. 8(</figref><i>d</i>)). In response to the detecting signal DET<b>2</b>, the selecting unit <b>64</b> turns the selecting signal OPTa to low level and turns the selecting signal OPTb to high level. In synchronization with the falling edge of the selecting signal OPTa, the signal generating unit <b>66</b><i>a </i>fixes the light volume adjusting signal ADJa to the turn-off level for turning off the fluorescent tube <b>46</b><i>a </i>(turn-off operation). Then, the fluorescent tube <b>46</b><i>a </i>goes out (<figref idref="DRAWINGS">FIG. 8(</figref><i>e</i>)). In response to the rising edge of the selecting signal OPTb, the signal generating unit <b>66</b><i>d </i>enhances the light volume adjusting signal ADJd steeply so that the fluorescent tube <b>46</b><i>d </i>increases in luminance as much as the minimum luminance of the fluorescent tube <b>46</b><i>a </i>(<figref idref="DRAWINGS">FIG. 8(</figref><i>f</i>)). This prevents the back light <b>32</b><i>b </i>from dropping in luminance due to the turning-off of the fluorescent tube <b>46</b><i>a. </i>That is, the back light <b>32</b><i>b </i>makes little variation in luminance when the fluorescent tube <b>46</b><i>a </i>is turned off.

0092Subsequently, while receiving the selecting signal OPTb of high level, the signal generating unit <b>66</b><i>d </i>outputs to the lighting control circuit <b>50</b><i>b </i>a light volume adjusting signal ADJd for adjusting the luminance of the fluorescent tube <b>46</b><i>d </i>so that the luminance gap signal GAP decreases. Consequently, the fluorescent tube <b>46</b><i>d </i>decreases in luminance (<figref idref="DRAWINGS">FIG. 8(</figref><i>g</i>)). With the decreasing luminance of the fluorescent tube <b>46</b><i>d, </i>the back light <b>32</b><i>b </i>also decreases in luminance (<figref idref="DRAWINGS">FIG. 8(</figref><i>h</i>)).

0093At time t<b>3</b>, the luminance of the fluorescent tube <b>46</b><i>d </i>reaches the minimum luminance, and the detecting part <b>56</b> outputs the detecting signal DET<b>1</b> (<figref idref="DRAWINGS">FIG. 8(</figref><i>i</i>)). In synchronization with the detecting signal DET<b>1</b>, the comparing unit <b>62</b> outputs the detecting signal DET<b>2</b> (<figref idref="DRAWINGS">FIG. 8(</figref><i>i</i>)). In response to the detecting signal DET<b>2</b>, the selecting unit <b>64</b> turns the selecting signal OPTb to low level and turns the selecting signal OPTc to high level. In synchronization with the falling edge of the selecting signal OPTb, the signal generating unit <b>66</b><i>d </i>fixes the light volume adjusting signal ADJd to the turn-off level for turning off the fluorescent tube <b>46</b><i>d </i>(turn-off operation). Then, the fluorescent tube <b>46</b><i>d </i>goes out (<figref idref="DRAWINGS">FIG. 8(</figref><i>j</i>)). In response to the rising edge of the selecting signal OPTc, the signal generating unit <b>66</b><i>e </i>enhances the light volume adjusting signal ADJe steeply so that the fluorescent tube <b>46</b><i>c </i>increases in luminance as much as the minimum luminance of the fluorescent tube <b>46</b><i>d </i>(<figref idref="DRAWINGS">FIG. 8(</figref><i>k</i>)). This prevents the back light <b>32</b><i>b </i>from dropping in luminance due to the turning-off of the fluorescent tube <b>46</b><i>d. </i>That is, the back light <b>32</b><i>b </i>makes little variation in luminance when the fluorescent tube <b>46</b><i>d </i>is turned off.

0094Subsequently, while receiving the selecting signal OPTc of high level, the signal generating unit <b>66</b><i>e </i>outputs to the lighting control circuit <b>50</b><i>c </i>a light volume adjusting signal ADJe for adjusting the luminance of the fluorescent tube <b>46</b><i>c </i>so that the luminance gap signal GAP decreases. Consequently, the fluorescent tube <b>46</b><i>c </i>decreases in luminance (<figref idref="DRAWINGS">FIG. 8(</figref><i>l</i>)). With the decreasing luminance of the fluorescent tube <b>46</b><i>c, </i>the back light <b>32</b><i>b </i>also decreases in luminance (<figref idref="DRAWINGS">FIG. 8(</figref><i>m</i>)).

0095At time t<b>4</b>, the luminance of the fluorescent tube <b>46</b><i>c </i>reaches the minimum luminance, and the detecting part <b>56</b> outputs the detecting signal DET<b>1</b> (<figref idref="DRAWINGS">FIG. 8(</figref><i>n</i>)). In synchronization with the detecting signal DET<b>1</b>, the comparing unit <b>62</b> outputs the detecting signal DET<b>2</b> (<figref idref="DRAWINGS">FIG. 8(</figref><i>n</i>)). Here, the back light <b>32</b><i>b </i>is at the minimum in luminance, so that the luminance indicated by the luminance detecting signal VD and the luminance indicated by the luminance adjusting signal LUM become equal. Then, the comparing unit <b>62</b> outputs a luminance gap signal GAP of 0 in magnitude. When the fluorescent tube <b>46</b><i>c </i>lit to the end becomes minimum in luminance, the brightness control circuit <b>48</b><i>b </i>makes no turn-off operation and keeps the fluorescent tube <b>46</b><i>c </i>lit at the minimum luminance.

0096As has been described, the liquid crystal display <b>28</b><i>b </i>of the present embodiment can offer the same effects as those of the foregoing first embodiment. Besides, in the present embodiment, the adjusting part <b>58</b><i>b </i>enhances the luminances of the fluorescent tubes <b>46</b><i>d </i>and <b>46</b><i>c </i>steeply in turning off the fluorescent tubes <b>46</b><i>a </i>and <b>46</b><i>d, </i>respectively. The back light <b>32</b><i>b </i>can thus be prevented from changing in luminance due to the turning-off of the fluorescent tubes <b>46</b><i>a </i>and <b>46</b><i>d </i>that are lit at the minimum luminances.

0097The fluorescent tubes <b>46</b><i>d </i>arranged in the centers of the reflectors <b>45</b> have a diameter smaller than the diameter of the outer fluorescent tubes <b>46</b><i>a </i>and <b>46</b><i>c. </i>Hence, the fluorescent tubes <b>46</b><i>d </i>of smaller parasitic capacitance can be increased in impedance. Consequently, even if relatively the same effective voltages are applied to all the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>d, </i>and <b>46</b><i>c, </i>the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>d, </i>and <b>46</b><i>c </i>can be made identical in the current that contributes to light emission. As a result, the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>d, </i>and <b>46</b><i>c </i>can be made identical in life, allowing extended life of the back light <b>32</b><i>b. </i>

0098In addition, since relatively the same effective voltages may be applied to the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>d, </i>and <b>46</b><i>c, </i>the signal generating units <b>66</b><i>a, </i><b>66</b><i>d, </i>and <b>66</b><i>e </i>can be made common in circuitry.

0099<figref idref="DRAWINGS">FIG. 9</figref> shows a third embodiment of the liquid crystal display of the present invention. The same elements as those of the first and second embodiments will be designated by identical reference numbers or symbols. Detailed description will be omitted of these elements. A liquid crystal display <b>28</b><i>c </i>includes a liquid crystal panel <b>30</b>, a back light <b>32</b><i>c, </i>and a case <b>34</b> for accommodating the liquid crystal panel <b>30</b> and the back light <b>32</b><i>c. </i>

0100The back light <b>32</b><i>c </i>includes a prism sheet <b>36</b>, a diffusion sheet <b>38</b>, a light guide plate <b>40</b>, a reflecting sheet <b>42</b>, and light emitting parts <b>44</b>C arranged on both sides of the light guide plate <b>40</b>, respectively.

0101The light emitting parts <b>44</b>C have a reflector <b>45</b> each. The light emitting part <b>44</b>C on the right of the light guide plate <b>40</b> in <figref idref="DRAWINGS">FIG. 9</figref> accommodates fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>g, </i>and <b>46</b><i>c. </i>The light emitting part <b>44</b>C on the left of the light guide plate <b>40</b> in <figref idref="DRAWINGS">FIG. 9</figref> accommodates fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>h, </i>and <b>46</b><i>c. </i>

0102The fluorescent tube <b>46</b><i>g </i>and the fluorescent tube <b>46</b><i>h </i>are identical to each other. The fluorescent tubes <b>46</b><i>g </i>and <b>46</b><i>h </i>are arranged in the centers of the respective reflectors <b>45</b>. All the minimum luminances and maximum luminances possible for the respective fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>g, </i><b>46</b><i>h, </i>and <b>46</b><i>c </i>to light at with stability are approximately the same.

0103The fluorescent tubes <b>46</b><i>g </i>and <b>46</b><i>h </i>have an internal gas pressure higher than that of the fluorescent tubes <b>46</b><i>a </i>and <b>46</b><i>c. </i>On this account, the fluorescent tubes <b>46</b><i>g </i>and <b>46</b><i>h </i>are higher in impedance than the fluorescent tubes <b>46</b><i>a </i>and <b>46</b><i>c. </i>Moreover, the fluorescent tubes <b>46</b><i>g </i>and <b>46</b><i>h, </i>arranged in the centers of the reflectors <b>45</b>, cause smaller parasitic capacitances with the reflectors <b>45</b> as in the second embodiment. Thus, the fluorescent tubes <b>46</b><i>g </i>and <b>46</b><i>h </i>are less prone to leak currents to the reflectors <b>45</b> than the fluorescent tubes <b>46</b><i>a </i>and <b>46</b><i>c </i>do.

0104Consequently, as in the second embodiment, the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>g, </i><b>46</b><i>h, </i>and <b>46</b><i>c </i>become identical in the current that contributes to light emission, provided that the same effective voltage is applied to all the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>g, </i><b>46</b><i>h, </i>and <b>46</b><i>c. </i>The result is that the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>g, </i><b>46</b><i>h, </i>and <b>46</b><i>c </i>become identical in life as with the first embodiment.

0105<figref idref="DRAWINGS">FIG. 10</figref> shows the details of control of the back light <b>32</b><i>c. </i>The back light <b>32</b><i>c </i>has a brightness control circuit <b>48</b><i>c, </i>two lighting control circuits <b>50</b><i>a </i>corresponding to the fluorescent tubes <b>46</b><i>a, </i>respectively, two lighting control circuits <b>50</b><i>b </i>corresponding to the fluorescent tubes <b>46</b><i>g </i>and <b>46</b><i>h, </i>respectively, two lighting control circuits <b>50</b><i>c </i>corresponding to the fluorescent tubes <b>46</b><i>c, </i>respectively, and an oscillating circuit <b>54</b>.

0106The lighting control circuits <b>50</b><i>a, </i><b>50</b><i>b, </i>and <b>50</b><i>c </i>apply relatively the same effective voltages to the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>g, </i><b>46</b><i>h, </i>and <b>46</b><i>c. </i>

0107The brightness control circuit <b>48</b><i>c </i>is composed of a detecting part <b>56</b> and an adjusting part <b>58</b><i>c. </i>The adjusting part <b>58</b><i>c </i>has a comparing unit <b>62</b>, a selecting unit <b>64</b><i>c, </i>and signal generating units <b>66</b><i>a, </i><b>66</b><i>g, </i><b>66</b><i>h, </i>and <b>66</b><i>j. </i>

0108The selecting unit <b>64</b><i>c </i>always outputs (keeps at high level) at least any one of selecting signals OPTa, OPTg, OPTh, and OPTc. The selecting signal OPTa is output to the signal generating unit <b>66</b><i>a. </i>The selecting signal OPTg is output to the signal generating unit <b>66</b><i>g </i>and the signal generating unit <b>66</b><i>h. </i>The selecting signal OPTh is output to the signal generating unit <b>66</b><i>h. </i>The selecting signal OPTc is output to the signal generating unit <b>66</b><i>j. </i>

0109Now, description will be given of the luminance adjustment of the back light <b>32</b><i>c </i>described above.

0110<figref idref="DRAWINGS">FIG. 11</figref> shows luminance variations of the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>g, </i><b>46</b><i>h, </i>and <b>46</b><i>c, </i>and the back light <b>32</b><i>c. </i>In this example, as with the first embodiment, the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>g, </i><b>46</b><i>h, </i>and <b>46</b><i>c </i>are lit at their respective maximum luminances in an initial state (at time t<b>0</b>). Besides, the selecting unit <b>64</b><i>c </i>outputs the selecting signal OPTa (keeps the selecting signal OPTa at high level).

0111At time t<b>1</b>, a luminance adjusting signal LUM indicating that the back light <b>32</b><i>c </i>is to be minimized in luminance is supplied, in which case the two fluorescent tube <b>46</b><i>a </i>gradually decrease in luminance (<figref idref="DRAWINGS">FIG. 11(</figref><i>a</i>)). With the decreasing luminances of the fluorescent tubes <b>46</b><i>a, </i>the back light <b>32</b><i>c </i>also decreases in luminance (<figref idref="DRAWINGS">FIG. 11(</figref><i>b</i>)). Since the output of the signal generating units <b>66</b><i>g, </i><b>66</b><i>h, </i>and <b>66</b><i>j </i>is fixed, the fluorescent tubes <b>46</b><i>g, </i><b>46</b><i>h, </i>and <b>46</b><i>c </i>have no change in luminance (<figref idref="DRAWINGS">FIG. 11(</figref><i>c</i>)).

0112At time t<b>2</b>, the luminances of the fluorescent tubes <b>46</b><i>a </i>reach the minimum luminances, and the detecting part <b>56</b> outputs the detecting signal DET<b>1</b> (<figref idref="DRAWINGS">FIG. 11(</figref><i>d</i>)). In synchronization therewith, the comparing unit <b>62</b> outputs the detecting signal DET<b>2</b> (<figref idref="DRAWINGS">FIG. 11(</figref><i>d</i>)). In response to the detecting signal DET<b>2</b>, the selecting unit <b>64</b><i>c </i>turns the selecting signal OPTa to low level and turns the selecting signal OPTc to high level. In synchronization with the falling edge of the selecting signal OPTa, the signal generating unit <b>66</b><i>a </i>fixes the light volume adjusting signal ADJa to a turn-off level for turning off the fluorescent tubes <b>46</b><i>a </i>(turn-off operation). Then, the two fluorescent tubes <b>46</b><i>a </i>go out (<figref idref="DRAWINGS">FIG. 11(</figref><i>e</i>)). In response to the rising edge of the selecting signal OPTc, the signal generating unit <b>66</b><i>j </i>enhances a light volume adjusting signal ADJj steeply so that the two fluorescent tubes <b>46</b><i>c </i>increase in luminance as much as the minimum luminances of the two fluorescent tubes <b>46</b><i>a </i>(<figref idref="DRAWINGS">FIG. 11(</figref><i>f</i>)). This prevents the back light <b>32</b><i>c </i>from dropping in luminance due to the turning-off of the two fluorescent tubes <b>46</b><i>a. </i>That is, the back light <b>32</b><i>c </i>makes little variation in luminance when the two fluorescent tubes <b>46</b><i>a </i>are turned off.

0113Subsequently, while receiving the selecting signal OPTc of high level, the signal generating unit <b>66</b><i>j </i>outputs a light volume adjusting signal ADJj for adjusting the luminances of the fluorescent tubes <b>46</b><i>c </i>so that the luminance gap signal GAP decreases. Consequently, the two fluorescent tubes <b>46</b><i>c </i>decrease in luminance (<figref idref="DRAWINGS">FIG. 11(</figref><i>g</i>)). With the decreasing luminances of the fluorescent tubes <b>46</b><i>c, </i>the back light <b>32</b><i>c </i>also decreases in luminance (<figref idref="DRAWINGS">FIG. 11(</figref><i>h</i>)).

0114At time t<b>3</b>, the luminances of the two fluorescent tubes <b>46</b><i>c </i>reach the minimum luminances, and the detecting part <b>56</b> outputs the detecting signal DET<b>1</b> (<figref idref="DRAWINGS">FIG. 11(</figref><i>i</i>)). In synchronization therewith, the comparing unit <b>62</b> outputs the detecting signal DET<b>2</b> (<figref idref="DRAWINGS">FIG. 11(</figref><i>i</i>)). In response to the detecting signal DET<b>2</b>, the selecting unit <b>64</b><i>c </i>turns the selecting signal OPTc to low level and turns the selecting signals OPTg and OPTh to high level. In synchronization with the falling edge of the selecting signal OPTc, the signal generating unit <b>66</b><i>j </i>fixes the light volume adjusting signal ADJj to a turn-off level for turning off the fluorescent tubes <b>46</b><i>c </i>(turn-off operation). Then, the two fluorescent tubes <b>46</b><i>c </i>go out (<figref idref="DRAWINGS">FIG. 11(</figref><i>j</i>)).

0115In response to the rising edge of the selecting signal OPTg, the signal generating unit <b>66</b><i>g </i>enhances a light volume adjusting signal ADJg steeply so that the fluorescent tube <b>46</b><i>g </i>increases in luminance as much as the minimum luminance of a single fluorescent tube <b>46</b><i>c </i>(<figref idref="DRAWINGS">FIG. 11(</figref><i>k</i>)). In response to the rising edge of the selecting signal OPTh, the signal generating unit <b>66</b><i>h </i>enhances a light volume adjusting signal ADJh steeply so that the fluorescent tube <b>46</b><i>h </i>increases in luminance as much as the minimum luminance of a single fluorescent tube <b>46</b><i>c </i>(<figref idref="DRAWINGS">FIG. 11(</figref><i>l</i>)). This prevents the back light <b>32</b><i>c </i>from dropping in luminance due to the turning-off of the two fluorescent tubes <b>46</b><i>c. </i>That is, the back light <b>32</b><i>c </i>makes little variation in luminance when the two fluorescent tubes <b>46</b><i>c </i>are turned off.

0116Subsequently, while receiving the selecting signal OPTg of high level, the signal generating unit <b>66</b><i>g </i>outputs a light volume adjusting signal ADJg for adjusting the luminance of the fluorescent tube <b>46</b><i>g </i>so that the luminance gap signal GAP decreases. Consequently, the fluorescent tube <b>46</b><i>g </i>decreases in luminance (<figref idref="DRAWINGS">FIG. 11(</figref><i>m</i>)). In the meantime, the signal generating unit <b>66</b><i>h, </i>while receiving the selecting signal OPTh of high level, outputs a light volume adjusting signal ADJh for adjusting the luminance of the fluorescent tube <b>46</b><i>h </i>so that the luminance gap signal GAP decreases. Consequently, the fluorescent tube <b>46</b><i>h </i>decreases in luminance (<figref idref="DRAWINGS">FIG. 11(</figref><i>n</i>)). With the decreasing luminances of the fluorescent tubes <b>46</b><i>g </i>and <b>46</b><i>h, </i>the back light <b>32</b><i>c </i>also decreases in luminance (<figref idref="DRAWINGS">FIG. 11(</figref><i>o</i>)).

0117At time t<b>4</b>, the luminances of the fluorescent tubes <b>46</b><i>g </i>and <b>46</b><i>h </i>reach the minimum luminances, and the detecting part <b>56</b> outputs the detecting signal DET<b>1</b> (FIG. <b>11</b>(<i>p</i>)). In synchronization therewith, the comparing unit <b>62</b> outputs the detecting signal DET<b>2</b> (<figref idref="DRAWINGS">FIG. 11(</figref><i>p</i>)). In response to the detecting signal DET<b>2</b>, the selecting unit <b>64</b><i>c </i>turns the selecting signal OPTg to low level. Here, the selecting signal OPTh is kept at high level. In synchronization with the falling edge of the selecting signal OPTg, the signal generating unit <b>66</b><i>g </i>fixes the light volume adjusting signal ADJg to a turn-off level for turning off the fluorescent tube <b>46</b><i>g </i>(turn-off operation). Then, the fluorescent tube <b>46</b><i>g </i>goes out (<figref idref="DRAWINGS">FIG. 11(</figref><i>q</i>)).

0118In response to the falling edge of the selecting signal OPTg, the signal generating unit <b>66</b><i>h </i>enhances the light volume adjusting signal ADJh steeply so that the fluorescent tube <b>46</b><i>h </i>increases in luminance as much as the minimum luminance of the fluorescent tube <b>46</b><i>g </i>(<figref idref="DRAWINGS">FIG. 11(</figref><i>r</i>)). This prevents the back light <b>32</b><i>c </i>from dropping in luminance due to the turning-off of the fluorescent tube <b>46</b><i>g. </i>That is, the back light <b>32</b><i>c </i>makes little variation in luminance when the fluorescent tube <b>46</b><i>g </i>is turned off.

0119At time t<b>5</b>, the luminance of the fluorescent tube <b>46</b><i>h </i>reaches the minimum luminance, and the detecting part <b>56</b> outputs the detecting signal DET<b>1</b> (<figref idref="DRAWINGS">FIG. 11(</figref><i>s</i>)). In synchronization therewith, the comparing unit <b>62</b> outputs the detecting signal DET<b>2</b> (<figref idref="DRAWINGS">FIG. 11(</figref><i>s</i>)). Here, the back light <b>32</b><i>c </i>is at the minimum in luminance, so that the luminance indicated by the luminance detecting signal VD and the luminance indicated by the luminance adjusting signal LUM become equal. Then, the comparing unit <b>62</b> outputs a luminance gap signal GAP of 0 in magnitude. When the fluorescent tube <b>46</b><i>h </i>lit to the end becomes minimum in luminance, the brightness control circuit <b>48</b><i>c </i>makes no turn-off operation and keeps the fluorescent tube <b>46</b><i>h </i>lit at the minimum luminance.

0120As has been described, the liquid crystal display <b>28</b><i>c </i>of the present embodiment can offer the same effects as those of the foregoing first and second embodiments. Besides, in the present embodiment, when the luminance adjusting signal LUM for minimizing the luminance of the back light is received, only a single fluorescent tube <b>46</b><i>h </i>is lit at its minimum luminance while the other fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>c, </i>and <b>46</b><i>g </i>are turned off. The maximum luminance possible for the back light <b>32</b><i>c </i>to output is for situations where the six fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>g, </i><b>46</b><i>h, </i>and <b>46</b><i>c </i>are lit at their maximum luminances. On this account, the minimum luminance of the back light <b>32</b><i>c </i>can be rendered one-sixth that of conventional back lights in which all the fluorescent tubes are lit together at the minimum luminances.

0121The fluorescent tubs <b>46</b><i>g </i>and <b>46</b><i>h </i>arranged in the centers of the reflectors <b>45</b> have an internal gas pressure higher than the internal gas pressure of the outer fluorescent tubes <b>46</b><i>a </i>and <b>46</b><i>c. </i>The fluorescent tubes <b>46</b><i>g </i>and <b>46</b><i>h </i>of smaller parasitic capacitance can thus be increased in impedance. Consequently, the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>g, </i><b>46</b><i>h, </i>and <b>46</b><i>c </i>can be made identical in the current that contributes to light emission, provided that relatively the same effective voltages are applied to all the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>g, </i><b>46</b><i>h, </i>and <b>46</b><i>c. </i>As a result, the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>g, </i><b>46</b><i>h, </i>and <b>46</b><i>c </i>can be made identical in life, allowing extended life of the back light <b>32</b><i>c. </i>

0122<figref idref="DRAWINGS">FIG. 12</figref> shows a fourth embodiment of the liquid crystal display of the present invention. The same elements as those of the first and second embodiments will be designated by identical reference numbers or symbols. Detailed description will be omitted of these elements. A liquid crystal display <b>28</b><i>d </i>includes a liquid crystal panel <b>30</b>, a case <b>34</b>, and a back light <b>32</b><i>d </i>of the present embodiment.

0123The back light <b>32</b><i>d </i>has light emitting parts <b>44</b>D arranged on both sides of a light guide plate <b>40</b>, respectively. In this embodiment, each light emitting part <b>44</b>D has reflectors <b>45</b><i>a, </i><b>45</b><i>b, </i>and <b>45</b><i>c </i>for accommodating fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c, </i>respectively. The brightness control circuit and the lighting control circuits are the same as in the second embodiment.

0124The reflectors <b>45</b><i>a, </i><b>45</b><i>b, </i>and <b>45</b><i>c </i>are identical to one another. The inner surfaces of the reflectors <b>45</b><i>a, </i><b>45</b><i>b, </i>and <b>45</b><i>c </i>are provided with reflecting parts <b>52</b><i>a, </i><b>52</b><i>b, </i>and <b>52</b><i>c </i>(reflecting coats) for reflecting the light emitted from the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c, </i>respectively. The reflectors <b>45</b><i>a, </i><b>45</b><i>b, </i>and <b>45</b><i>c </i>are arranged with their apertures toward the sides of the light guide plate <b>40</b>.

0125The physical relationship between the fluorescent tube <b>46</b><i>a </i>and the reflector <b>45</b><i>a, </i>the physical relationship between the fluorescent tube <b>46</b><i>b </i>and the reflector <b>45</b><i>b, </i>and the physical relationship between the fluorescent tube <b>46</b><i>c </i>and the reflector <b>45</b><i>c </i>are identical. For this reason, all the parasitic capacitances occurring between the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c, </i>and the reflectors <b>45</b><i>a, </i><b>45</b><i>b, </i>and <b>45</b><i>c </i>accommodating the respective fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c </i>are the same.

0126The areas of parts of the reflectors <b>45</b><i>a, </i><b>45</b><i>b, </i>and <b>45</b><i>c </i>where there is no obstacle between the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c </i>and the reflectors <b>45</b><i>a, </i><b>45</b><i>b, </i>and <b>45</b><i>c, </i>and they face to each other, respectively, are greater than in the cases where a single reflector accommodates three fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c. </i>This allows improved conductive efficiency of dissipating the heat produced by the lit fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c </i>to the reflectors <b>45</b><i>a, </i><b>45</b><i>b, </i>and <b>45</b><i>c. </i>

0127As has been described, the liquid crystal display <b>28</b><i>d </i>of the present embodiment can offer the same effects as those of the foregoing second embodiment. In addition, the present embodiment provides the reflectors <b>45</b><i>a, </i><b>45</b><i>b, </i>and <b>45</b><i>c </i>for accommodating the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c, </i>respectively. The heat produced by the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c </i>can thus be dissipated with higher efficiency.

0128Accommodating the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c </i>in the reflectors <b>45</b><i>a, </i><b>45</b><i>b, </i>and <b>45</b><i>c, </i>respectively, equalizes the parasitic capacitances between the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c </i>and the reflectors <b>45</b><i>a, </i><b>45</b><i>b, </i>and <b>45</b><i>c. </i>Consequently, the currents flowing through the respective fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c </i>can be made identical so that the fluorescent tubes <b>46</b><i>a, </i><b>46</b><i>b, </i>and <b>46</b><i>c </i>become equal in life. As a result, the life of the back light <b>32</b><i>d </i>can be prolonged.

0129Incidentally, the foregoing first to fourth embodiments have dealt with the cases where the light emitting parts (reflectors and fluorescent tubes) are arranged on both sides of the light guide plate <b>40</b>, respectively. However, the present invention is not limited to such embodiments. For example, a light emitting part may be arranged on either side of the light guide plate, as shown in <figref idref="DRAWINGS">FIG. 13</figref>.

0130The invention is not limited to the above embodiments and various modifications may be made without departing from the spirit and scope of the invention. Any improvement may be made in part or all of the components.